Method of manufacturing silicon carbide semiconductor device

ABSTRACT

A carbon film deposition apparatus includes an evaporator for evaporating an oxygen-containing hydrocarbon. The carbon film deposition apparatus also includes a gas inlet pipe for introducing the oxygen-containing hydrocarbon gas evaporated in the evaporator. The carbon film deposition apparatus further includes a deposition furnace for depositing a carbon protection film over all surfaces of a wafer by pyrolyzing the oxygen-containing hydrocarbon gas introduced through the gas inlet pipe.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of priorityunder 35 U.S.C. §120 from U.S. Ser. No. 12/165,841, filed Jul. 1, 2008,the entire contents of which are incorporated herein by reference. U.S.Ser. No. 12/165,841 claims the benefit of priority under 35 U.S.C. §119from Japanese Patent Application Nos. 2007-209282, filed Aug. 10, 2007and 2008-073440, filed Mar. 21, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods of manufacturing a siliconcarbide semiconductor device.

2. Description of the Prior Art

Silicon carbide (SiC) allows for manufacturing a silicon carbidesemiconductor device that has a higher breakdown voltage characteristiccompared to using conventional silicon (Si), and has been expected as amaterial for a high power semiconductor device of the next generation.In manufacturing a silicon carbide semiconductor device from suchsilicon carbide, in order to control its conductivity-type andconductivity, an n-type or a p-type impurity is ion-implanted into asilicon carbide wafer that consists of silicon carbide layers formed byepitaxial crystal growth on a silicon carbide substrate. After the ionimplantation, in order to activate the implanted ion and remove crystaldefects having been created owing to the ion implantation, the siliconcarbide wafer is processed by an annealing treatment in which the waferis exposed to a hot atmosphere of an inert gas such as argon (Ar). Suchannealing treatment of a silicon carbide wafer is preferably performedat a temperature as high as possible, in most cases at above 1500° C.,desirably at above 1600° C., for stabilizing the characteristic thereof.

Annealing a silicon carbide wafer at such a high temperature, however,creates asperity called step bunching on the surface of the siliconcarbide wafer. The reason for the creation of step bunching is asfollows.

A silicon carbide wafer is typically obtained by forming silicon carbidelayers by epitaxial crystal growth on a silicon carbide substrate. Theepitaxial crystal growth is performed in such a way that the crystalaxis is inclined approximately four or eight degrees with respect to theC-axis that is orthogonal to the (0001) crystal plane, in order toprevent crystals of, for example, the 6H- and 4H-types from mixedlygrowing on the same crystal surface.

When the silicon carbide wafer thus crystallized with its crystal axisbeing inclined is exposed to such a high temperature as in annealingtreatments, silicon (Si) and carbon (C), which are the constituentelements thereof, evaporate from the surface of the silicon carbidewafer. In the evaporation, since silicon and carbon have differentevaporating conditions and the crystal axis is inclined, the evaporationrates of silicon and carbon differ from each other in the surface of thesilicon carbide wafer. As a result, step bunching is created on thesilicon carbide wafer surface.

The step bunching thus created becomes an obstacle in forming a gateoxide film on the silicon carbide wafer having been annealed and furtherbecomes an obstacle in forming a gate electrode on the gate oxide film.For example, there is a possibility of contactability reducing and ofthe leakage characteristic deteriorating due to asperity in the boundarysurfaces between the silicon carbide wafer and the gate oxide film, orthe gate oxide film and the gate electrode.

For that reason, to prevent or reduce step bunching has been a majorproblem for quality stability of silicon carbide semiconductor devicesand for improvement in their manufacturing yields.

As for a method of preventing or reducing such step bunching, atechnology has been disclosed in, for example, Patent Document 1(Japanese Patent No. 3760688) in which a diamond-like carbon film or anorganic film is formed on the surface of a silicon carbide wafer as aprotection film that prevents evaporation of silicon and carbon duringan annealing treatment.

Another technology has been disclosed in Patent Document 2 (JapanesePatent Application Publication No. 2005-353771) in which a carbon filmis formed on an ion-implanted side of a silicon carbide wafer by asputtering method as a protection film that prevents evaporation ofsilicon and carbon during an annealing treatment.

In the method of Patent Document 1, however, a carbonized resist is usedas a protection film. A resist generally includes numbers of elementsother than carbon and hydrogen for enhancing its optical activity andcontactability. These elements other than carbon and hydrogen remainbehind as contaminants in the protection film formed from the carbonizedresist. For that reason, the contaminants evaporate or scatter during anannealing treatment, to be a source of contamination to the siliconcarbide semiconductor device.

Moreover, in the method of Patent Document 2, while a carbon film formedby a sputtering method is used as a protection film, a sputtering methodalso sputters part of materials (for example, metal materials such asaluminum and stainless steel) that make up the sputtering apparatus.Accordingly, these sputtered materials scatter as contaminants, to be asource of contamination to the silicon carbide semiconductor device.

Furthermore, in the methods of Patent Documents 1 and 2, a protectionfilm is formed on one surface of a silicon carbide wafer, to be morespecific, on a side that is ion-implanted with an impurity, of thesilicon carbide wafer. When a protection film is thus formed on only onesurface of a silicon carbide wafer, unbalanced thermal stress is createdin the silicon carbide wafer during an annealing treatment due to atemperature gradient produced therein and difference between the thermalexpansion coefficients of the silicon carbide wafer and the protectionfilm, which results in an increase of crystal defects in the siliconcarbide wafer.

Such ingress of contaminants and increase of crystal defects are factorsthat contribute to unstable quality of silicon carbide semiconductordevices as well as reduction in manufacturing yields thereof.

SUMMARY OF THE INVENTION

The present invention is made to solve the above problems, and an objectof the invention is to provide a method of forming on a silicon carbidewafer a protection film that is of extremely low contamination andcauses no unbalanced thermal stress in the silicon carbide wafer forpreventing creation of step bunching during an annealing treatment, andto thereby obtain a method of manufacturing a silicon carbidesemiconductor device, which achieves improvements in its qualitystability and manufacturing yields.

A method of manufacturing a silicon carbide semiconductor device,according to the present invention includes a step of ion-implanting animpurity into a surface of a silicon carbide wafer; a step of forming acarbon protection film of a predetermined thickness over the entiresurface of the silicon carbide wafer having been ion-implanted with theimpurity, by a chemical vapor deposition method that deposits a film bypyrolyzing a hydrocarbon gas; and a step of annealing the siliconcarbide wafer having been formed with the carbon protection film.

According to the invention, a method of manufacturing a silicon carbidesemiconductor device includes a step of forming a carbon protection filmof a predetermined thickness over the entire surface of a siliconcarbide wafer having been ion-implanted with an impurity, using achemical vapor deposition method that deposits a film by pyrolyzing ahydrocarbon gas. Thereby, a carbon protection film of high purity andhigh quality can be obtained, which prevents the silicon carbidesemiconductor device from being contaminated. Moreover, since unbalancedthermal stress created in the silicon carbide wafer can also besuppressed, crystal defects associated with the thermal stress, in thesilicon carbide wafer are prevented from increasing. Therefore, themethod of manufacturing a silicon carbide semiconductor device can beobtained that achieves improvements in its quality stability andmanufacturing yields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one process in a method of manufacturing asilicon carbide semiconductor device, according to Embodiments 1 through4 of the present invention;

FIG. 2 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 3 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 4 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 5 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 6 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 7 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 8 is an illustration of one process in the method of manufacturingthe silicon carbide semiconductor device, according to Embodiments 1through 4 of the invention;

FIG. 9 is a schematic view illustrating a deposition apparatus accordingto Embodiments 2 of the invention;

FIG. 10 is a schematic view illustrating a deposition apparatusaccording to Embodiment 3 of the invention; and

FIG. 11 is a schematic view illustrating an annealing apparatusaccording to Embodiment 5 of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

A method of manufacturing a silicon carbide semiconductor deviceaccording to Embodiment 1 of the present invention will be described.Here, a method of manufacturing a power metal-oxide semiconductorfield-effect transistor (power MOSFET) is explained as an example withreference to FIGS. 1 through 8.

Firstly, an n⁻-type (a first conductivity-type) silicon carbide layer 2,which is made of silicon carbide, is formed on one surface of an n⁺-type(a first conductivity-type) semiconductor substrate 1 by epitaxialcrystal growth, as shown in FIG. 1. As the semiconductor substrate 1,for example, an n⁺-type silicon carbide substrate is preferable. Asilicon carbide wafer is made up of the semiconductor substrate 1 andthe silicon carbide layer 2.

Next, a p-type (a second conductivity-type) well region 3 is formed byion-implanting an impurity into a surface of the silicon carbide waferusing a resist as a mask for the well region. To be more specific, theimpurity is ion-implanted selectively into a portion outside apredetermined radius, on the surface of the silicon carbide layer 2constituting the silicon carbide wafer, as shown in FIG. 2. As animpurity to become a p-type in the silicon carbide layer 2, for example,boron (B) or aluminum (Al) is given. The resist is removed after the ionimplantation.

Next, an n-type (a first conductivity-type) source region 4 is formed byion-implanting an impurity selectively into the surface of the wellregion 3 using a resist as a mask for the source region, as shown inFIG. 2. As an impurity to become an n-type in the well region 3, forexample, phosphorus (P) or nitrogen (N) is given. After the ionimplantation, the resist is removed.

Next, a p⁺-type (a second conductivity-type) contact region 5 is formedcontiguous with the outer periphery of the source region 4 byion-implanting a p-type (a second conductivity-type) impurity into thesurface of the well region 3 using a resist as a mask for the contactregion, as shown in FIG. 2. A concentration of the impurity in thecontact region 5 is set here so as to be relatively higher than that inthe well region 3. As an impurity to become a p-type in the well region3, for example, boron (B) or aluminum (Al) is given. After the ionimplantation, the resist is removed.

Next, a carbon protection film 6 is formed over the entire surface ofthe silicon carbide wafer by a chemical vapor deposition (CVD) method orthe like using a hydrocarbon gas such as acethylene (C₂H₂), methane(CH₄), and propane (C₃H₈), as shown in FIG. 3. To be more specific, theinside of the deposition apparatus performing the CVD is heated to atemperature ranging from 900° C. to 1000° C. under atmospheric orreduced pressure while an inert gas such as argon (Ar) is beingintroduced as a carrier gas to flow into the deposition apparatus. Theheating time is approximately twenty minutes. After that, thehydrocarbon gas of approximately 10% concentration is introduced intothe flowing carrier gas, and then the hydrocarbon gas is pyrolyzed.Thereby, the carbon protection film 6 is formed to a predeterminedthickness over the entire surface of the silicon carbide wafer. Thepredetermined thickness here is preferable to be from one nanometer,above which the effect of the protection film is brought about, to onethousand nanometers, under which cracks due to the thermal load imposingthe silicon carbide wafer and to the temperature difference arisingtherein are not produced in the protection film. It is more preferableto be ten to five hundred nanometers, in which range the thickness iseasy to control. Since a hydrocarbon gas of high purity can be thus usedin a CVD method or the like, the carbon protection film 6 is formed tobe of high quality and high purity with few contaminants. The highquality here implies an advanced state of graphitization. Moreover,since the carbon protection film 6 is formed over the entire surface ofthe silicon carbide wafer, the film formation can be performed in abatch operation. In other words, the film formation can be performed fora plurality of silicon carbide wafers all together by placing themwithin a deposition apparatus at intervals such that the carbonprotection film 6 can be uniformly formed over the entire surface ofeach silicon carbide wafer, therefore improving throughput of the filmformation.

Next, the silicon carbide wafer having been deposited with the carbonprotection film 6 over the entire surface thereof is placed into anannealing apparatus, to be annealed in an inert gas atmosphere. To bemore specific, in an argon atmosphere, the wafer, after being pre-heatedat approximately 1000° C., is heated for a predetermined time(approximately thirty minutes) at a predetermined temperature (above1500° C., preferably around 1700° C.), and then cooled immediately.Thereby, the implanted ion is electrically activated, and crystaldefects created owing to the ion implantation are removed. After theannealing treatment, the carbon protection film 6 is removed by beingexposed to an oxygen atmosphere at approximately 950° C. for aboutthirty minutes. Otherwise, the carbon protection film 6 can also beremoved by ashing using oxygen plasma that is employed for resistremoval. During the annealing treatment, since the silicon carbide waferis thus covered with the carbon protection film 6 over the entiresurface thereof, no step bunching is created on the surface of thesilicon carbide wafer. Measurement of asperity on the surface of thesilicon carbide wafer by atomic force microscopy (AFM) after removal ofthe carbon protection film 6 revealed that, whereas asperity to theextent of several dozens nanometers was created over the entire surfaceof a silicon carbide wafer in a case without the carbon protection film6, asperity less than one nanometer was created in the case with thecarbon protection film 6 according to the invention, whereby the effectof the film was verified. Moreover, since the carbon protection film 6that is of high purity with few contaminants is formed, the siliconcarbide semiconductor device does not become contaminated. Furthermore,since the carbon protection film 6 is formed over the entire surface ofthe silicon carbide wafer, unbalanced thermal stress created thereinduring the annealing treatment is relieved, so that distortion createdin the silicon carbide wafer is reduced. For that reason, crystaldefects created in the silicon carbide wafer do not increase.

Next, a gate oxide film 7 made of silicon dioxide (SiO₂) is formed onone surface of, specifically, on the ion-implanted side of the siliconcarbide wafer from which the carbon protection film 6 has been removedby thermal oxidation, as shown in FIG. 4. The gate oxide film 7 thusformed in this process is a thermal oxidation film.

Next, a polysilicon film is temporarily formed on the gate oxide film 7by a CVD method. Then, a gate electrode 8 is formed, after anunnecessary portion of the polysilicon film is removed by a wet etchingmethod or a dry etching method such as a reactive ion etching (RIE)using a resist as a mask for the gate electrode, as shown in FIG. 5.

Next, an interlayer insulation film 9 made of silicon dioxide (SiO₂) isformed on the gate oxide film 7 and the gate electrode 8 by a CVD methodusing tetraethoxysilane (TEOS) gas, as shown in FIG. 6. The interlayerinsulation film 9 formed in this process is a TEOS oxide film.

Next, the interlayer insulation film 9 and the gate oxide film 7 areremoved by a wet etching method or a dry etching method such as a RIEusing a resist as a mask for exposing the contact region 5 and part ofthe source region 4, as shown in FIG. 7. After that, the resist is alsoremoved.

Next, a conductive film is formed on the contact region 5 and the partof the source region 4 that have been exposed by removing the interlayerinsulation film 9 and the gate oxide film 7, by a physical vapordeposition method (PVD Method) such as a sputtering method. After that,a source electrode 10 (a first main electrode) is formed on the contactregion 5 and the part of the source region 4 by removing an unnecessaryportion of the conductive film having been formed on the interlayerinsulation film 9 by a wet etching method or a dry etching method suchas a RIE using a resist as a mask for the source electrode, as shown inFIG. 8. The source electrode 10 is electrically connected with thecontact region 5 and the source region 4. Nickel (Ni) or aluminum (Al)is given as a material for the source electrode 10.

Lastly, a drain electrode 11 (a second main electrode) made of aconductive film is formed on the silicon carbide wafer surface oppositeto the source electrode by a PVD method such as a sputtering method, asshown in FIG. 8. Nickel (Ni) or aluminum (Al) is given as a material forthe drain electrode 11.

Through the processes described above, the main part of the MOSFET iscompleted that is the silicon carbide semiconductor device according toEmbodiment 1 of the invention.

A method of manufacturing a silicon carbide semiconductor device,according to Embodiment 1 of the invention includes a process of forminga carbon protection film of a predetermined thickness over the entiresurface of a silicon carbide wafer having been ion-implanted with animpurity by a CVD method that deposits a film by pyrolyzing ahydrocarbon gas. Since the carbon protection film 6 that is of highquality and high purity with few contaminants can thereby be obtained,the silicon carbide semiconductor device can be prevented from beingcontaminated. Moreover, since unbalanced thermal stress created in thesilicon carbide wafer can be suppressed, crystal defects, associatedwith the thermal stress, in the silicon carbide wafer can also beprevented from increasing. Therefore, the method of manufacturing asilicon carbide semiconductor device can be obtained that achievesimprovements in its quality stability and manufacturing yields.

Embodiment 2

In Embodiment 1, the carbon protection film 6 of high quality and highpurity is formed over the entire surface of a silicon carbide wafer bypyrolyzing a hydrocarbon gas such as acethylene or methane at atemperature ranging from 900° C. to 1000° C. Moreover, it is assumed inEmbodiment 1 that the pyrolysis of the hydrocarbon gas and the formationof the carbon protection film 6 are performed in a CVD depositionapparatus at the same time and in the same process. In contrast,Embodiment 2 is characterized in that a CVD deposition apparatus isconstituted with a gas pyrolysis furnace and a deposition furnace on theupstream and the downstream sides, respectively, with respect to ahydrocarbon gas flow, to perform pyrolysis of the hydrocarbon gas andformation of a carbon protection film 6 in separate processes in therespective furnaces. Hereinafter, a method of forming the carbonprotection film 6, according to Embodiment 2 will be explained withreference to the illustration of the deposition apparatus shown in FIG.9. In Embodiment 2, processes other than that of forming the carbonprotection film 6 by pyrolyzing the hydrocarbon gas are the same asthose explained in Embodiment 1; their explanations are thereforeomitted.

A deposition apparatus 20 for forming the carbon protection film 6 isillustrated in FIG. 9. The deposition apparatus 20 is provided with agas pyrolysis furnace 21 for pyrolyzing a hydrocarbon gas such asacethylene and methane; a deposition furnace 22 for depositing thecarbon protection film 6 over the entire surface of a silicon carbidewafer; a connection pipe 23 for introducing, from the gas pyrolysisfurnace 21 into the deposition furnace 22, the gas having been pyrolyzedfrom the hydrocarbon gas; a gas inlet pipe 24 for introducing thehydrocarbon gas into the gas pyrolysis furnace 21; and an exhaust gaspipe 25 for exhausting the pyrolyzed hydrocarbon gas in the depositionfurnace 22.

The gas pyrolysis furnace 21 is provided with a heater 26 and heatspreaders 27 for uniformly heating the inside of the gas pyrolysisfurnace 21 by receiving heat from the heater 26. The heater 26 isdisposed around the inner periphery of the gas pyrolysis furnace 21 soas not to be directly exposed to the hydrocarbon gas. The heat spreaders27 are made of silica glass or ceramics and disposed in the gaspyrolysis furnace 21. The gas pyrolysis furnace 21 thus configureduniformly transfers the heat from the heater 26 to the hydrocarbon gasthrough the heat spreaders 27, enabling complete pyrolysis of thehydrocarbon gas.

The deposition furnace 22 is provided with a heater 27 disposed aroundthe inner periphery thereof so as not to be directly exposed to thepyrolyzed hydrocarbon gas and a wafer boat, not shown in the figure, onwhich silicon carbide wafers are placed for batch operation so that thecarbon protection film 6 can be formed over the entire surface of eachsilicon carbide wafer.

The carbon protection film 6 is formed in the following way by suchdeposition apparatus 20.

An inert gas such as argon (Ar) is introduced as a carrier gas into theinside of the deposition apparatus 20 through the gas inlet pipe 24. Thepressure inside the deposition apparatus 20 is controlled by adjustingthe rate of exhaust gas at the exhaust gas pipe 25 so as to be anatmospheric or a reduced pressure. The insides of the gas pyrolysisfurnace 21 and the deposition furnace 22 are heated for about twentyminutes to a temperature of approximately 800° C. and to a temperatureranging from 500° C. to 800° C., respectively, under the atmospheric orthe reduced pressure in the deposition apparatus 20. After that, ahydrocarbon gas such as acethylene or methane is introduced at aconcentration of approximately 10% into the flowing carrier gas, to bepyrolyzed in the gas pyrolysis furnace 21. The hydrocarbon gas may beintroduced to flow into the carrier gas from the beginning. Thehydrocarbon gas is almost completely pyrolyzed due to the heat spreaders27. The hydrocarbon gas having been pyrolyzed in the gas pyrolysisfurnace 21 is then introduced into the deposition furnace 22 through theconnection pipe 23. The pyrolyzed hydrocarbon gas introduced into thedeposition furnace 22 is deposited over the entire surface of eachsilicon carbide wafer placed in the deposition furnace 22, so that thecarbon protection film 6 that is of high quality and high purity withfew contaminants is formed to a predetermined thickness. Thepredetermined thickness is preferable to be from one nanometer, abovewhich the effect of the protection film is brought about, to onethousand nanometers, under which cracks due to the thermal load imposingthe silicon carbide wafer and to the temperature difference arisingtherein are not produced in the protection film. It is more preferableto be ten to five hundred nanometers, in which range the thickness iseasy to control. In addition, the high quality implies an advanced stateof graphitization. After the carbon protection film 6 is formed to thepredetermined thickness, the gas inside the deposition apparatus 20 isimmediately exhausted, and at the same time, the inside of thedeposition apparatus 20 is cooled down to a temperature at which thesilicon carbide wafer can be removed therefrom.

According to Embodiment 2, since a deposition apparatus is separatedinto the gas pyrolysis furnace 21 and the deposition furnace 22, and aprocess of pyrolyzing a hydrocarbon gas such as acethylene or methane isprovided in advance before the carbon protection film 6 is formed, thecarbon protection film 6 that has an effect equal to that of Embodiment1 can be formed at a temperature lower than that therein. Therefore, amethod of manufacturing a silicon carbide semiconductor device can beobtained that achieves improvements in its quality stability andmanufacturing yields.

Moreover, if the inside the deposition furnace 22 is set at atemperature, for example, equal to that in Embodiment 1, the carbonprotection film 6 can be obtained that is of higher quality,specifically, of more advanced graphitization compared to Embodiment 1.Therefore, a method of manufacturing a silicon carbide semiconductordevice can be obtained that achieves further improvements in its qualitystability and manufacturing yields.

Embodiment 3

While, in Embodiment 1 and Embodiment 2, the cases of using ahydrocarbon gas such as acethylene, methane, and propane has beenexplained as examples, a lower alcohol such as ethyl alcohol (C₂H₅OH:ethanol) and methyl alcohol (CH₃OH: methanol), which is one ofoxygen-containing hydrocarbons, can also be used. Here, a method ofmanufacturing a silicon carbide semiconductor device using ethanol ormethanol will be described for a case with a power MOSFET as an example.The method of manufacturing the power MOSFET, except for a process offorming a carbon protection film, is the same as that shown in FIGS. 1through 8 in Embodiment 1; hence the description is made with referenceto FIGS. 1 through 8.

Firstly, an n⁻-type (a first conductivity-type) silicon carbide layer 2,which is made of silicon carbide, is formed on one surface of an n⁺-type(a first conductivity-type) semiconductor substrate 1 by epitaxialcrystal growth, as shown in FIG. 1. As the semiconductor substrate 1,for example, an n⁺-type silicon carbide substrate is preferable. Asilicon carbide wafer is made up of the semiconductor substrate 1 andthe silicon carbide layer 2.

Next, a p-type (a second conductivity-type) well region 3 is formed byion-implanting an impurity into a surface of the silicon carbide waferusing a resist as a mask for the well region. To be more specific, theimpurity is ion-implanted selectively into a portion outside apredetermined radius, on the surface of the silicon carbide layer 2constituting the silicon carbide wafer, as shown in FIG. 2. As animpurity to become a p-type in the silicon carbide layer 2, for example,boron (B) or aluminum (Al) is given. The resist is removed after the ionimplantation.

Next, an n-type (a first conductivity-type) source region 4 is formed byion-implanting an impurity selectively into the surface of the wellregion 3 using a resist as a mask for the source region, as shown inFIG. 2. As an impurity to become an n-type in the well region 3, forexample, phosphorus (P) or nitrogen (N) is given. After the ionimplantation, the resist is removed.

Next, a p⁺-type (a second conductivity-type) contact region 5 is formedcontiguous with the outer periphery of the source region 4 byion-implanting a p-type (a second conductivity-type) impurity into thesurface of the well region 3 using a resist as a mask for the contactregion, as shown in FIG. 2. A concentration of the impurity in thecontact region 5 is set here so as to be relatively higher than that inthe well region 3. As an impurity to become a p-type in the well region3, for example, boron (B) or aluminum (Al) is given. After the ionimplantation, the resist is removed.

Next, a carbon protection film 6 is formed over the entire surface ofthe silicon carbide wafer by a CVD method or the like using anoxygen-containing hydrocarbon gas such as ethanol and methanol, as shownin FIG. 3.

To be more specific, while an inert gas such as argon (Ar) is beingintroduced as a carrier gas to flow into the deposition apparatusperforming a CVD, the inside of the deposition apparatus is heated to atemperature ranging from 850° C. to 1000° C. under a reduced pressurebelow 1.33×10⁴ Pa (100 Torr), preferably below 6.67×10³ Pa (50 Torr),more preferably below 1.33×10³ Pa (10 Torr). The heating time isapproximately twenty minutes. After that, with the carrier gas beingshut off or flowing, the oxygen-containing hydrocarbon gas is fed, andthen the hydrocarbon gas is pyrolyzed under the reduced pressurementioned above, whereby the carbon protection film 6 is formed to apredetermined thickness over the entire surface of the silicon carbidewafer. The predetermined thickness here is preferable to be from onenanometer, above which the effect of the protection film is broughtabout, to one thousand nanometers, under which cracks due to the thermalload imposing the silicon carbide wafer and to the temperaturedifference arising therein are not produced in the protection film. Itis more preferable to be ten to five hundred nanometers, in which rangethe thickness is easy to control. Since a hydrocarbon gas of high puritycan be thus used in a CVD method or the like, the carbon protection film6 is formed to be of high quality and high purity with few contaminants.The high quality here implies an advanced state of graphitization.Moreover, since the carbon protection film 6 is formed over the entiresurface of the silicon carbide wafer, the film formation can beperformed in a batch operation. In other words, the film formation canbe performed for a plurality of silicon carbide wafers all together byplacing them within a deposition apparatus at intervals such that thecarbon protection film 6 can be uniformly formed over the entire surfaceof each silicon carbide wafer, therefore improving throughput of thefilm formation.

Here, a deposition apparatus suitable to form the carbon protection film6 is illustrated in FIG. 10 as an example in Embodiment 3.

Referring to FIG. 10, a deposition apparatus 30 is for forming thecarbon protection film 6. The deposition apparatus 30 is provided with agas inlet pipe 31 for introducing an oxygen-containing hydrocarbon gassuch as ethanol and methanol into the deposition apparatus; a depositionfurnace 32 for forming the carbon protection film 6 over the entiresurface of a silicon carbide wafer; an exhaust gas pipe 33 forexhausting the pyrolyzed hydrocarbon gas; a heater 34 disposed aroundthe periphery of the deposition furnace 32 so as not to be directlyexposed to the hydrocarbon gas, for pyrolyzing the hydrocarbon gas; anda substrate holder 35 for holding silicon carbide wafers in such a waythat the carbon protection film 6 can be formed over the entire surfaceof each wafer in a batch operation. It is conceivable that the substrateholder 35 holds the silicon carbide wafers by supporting theirperipheries, for example, at three points of each periphery. Inaddition, the substrate holder 35 has a function equal to that of thewafer boat for silicon carbide wafers being placed thereon, which ismentioned in Embodiment 1 and Embodiment 2.

Formation of the carbon protection film 6 by the deposition apparatus 30is as follows.

An inert gas such as argon (Ar) is introduced as a carrier gas into theinside of the deposition furnace 32 through the gas inlet pipe 31. Thepressure inside the deposition furnace 32 is controlled by adjusting therate of exhaust gas at the exhaust gas pipe 33 so as to be in a reducedpressure state (below 1.33×10⁴ Pa (100 Torr), preferably below 6.67×10³Pa (50 Torr), more preferably below 1.33×10³ Pa (10 Torr)). The insideof the deposition furnace 32 is heated by the heater 34 for about thirtyminutes to a temperature ranging from 800° C. to 1000° C. under thecondition of the above-mentioned reduced pressure. At the time when theinside of the deposition furnace 32 reaches a predetermined temperature,the carrier gas flow is shut off. Then, the oxygen-containinghydrocarbon gas such as ethanol and methanol is introduced through thegas inlet pipe 31 into the deposition furnace 32, and the inside thereofis adjusted to a predetermined pressure, for example, to theabove-mentioned reduced pressure. The oxygen-containing hydrocarbon gasintroduced into the deposition furnace 32 is pyrolyzed thereinside, sothat the carbon protection film 6 that is of high quality and highpurity with few contaminants is formed to the predetermined thicknessover the entire surface of each of a plurality of such silicon carbidewafers placed on the substrate holder 35. The predetermined thicknesshere, as mentioned before, is preferable to be from one nanometer, abovewhich the effect of the protection film is brought about, to onethousand nanometers, under which cracks due to the thermal load imposingthe silicon carbide wafer and to the temperature difference arisingtherein are not produced in the protection film. It is more preferableto be ten to five hundred nanometers, in which range the thickness iseasy to control. In addition, the high quality implies an advanced stateof graphitization. After the carbon protection film 6 of thepredetermined thickness is formed, the gas inside the depositionapparatus 32 is immediately exhausted, and at the same time, the carriergas is introduced again and the inside thereof is cooled down to atemperature at which the silicon carbide wafers can be removedtherefrom. In this way, the carbon protection film 6 of high quality andhigh purity with few contaminants is formed to the predeterminedthickness over the entire surface of each silicon carbide wafer. Inaddition, the configuration of the deposition apparatus 30 shown in FIG.10 can also be applied to formation of the carbon protection film 6shown in Embodiment 1. It is noted that the configuration of thedeposition apparatus 30 shown in FIG. 10 is just an example; hence, nospecific limitation is imposed to the configuration. A depositionapparatus may be provided with a configuration equivalent to that of thedeposition apparatus 30 shown in FIG. 10.

Next, the silicon carbide wafer having been deposited with the carbonprotection film 6 over the entire surface thereof is placed into anannealing apparatus, to be annealed in an inert gas atmosphere.

To be more specific, in an argon atmosphere, the wafer, after beingpre-heated at approximately 1000° C., is heated for a predetermined time(approximately 30 minutes) at a predetermined temperature (above 1500°C., preferably around 1700° C.), and then cooled immediately. Thereby,the implanted ion is electrically activated, and crystal defects createdby the ion implantation are removed.

After the annealing treatment, the carbon protection film 6 is removedby being exposed to an oxygen atmosphere at approximately 950° C. forabout thirty minutes. Otherwise, the carbon protection film 6 can alsobe removed by ashing using oxygen plasma that is employed for resistremoval.

During the annealing treatment, since the silicon carbide wafer is thuscovered with the carbon protection film 6 deposited over the entiresurface thereof, no step bunching is created on the silicon carbidewafer surface. Measurement of asperity on the surface of the siliconcarbide wafer by atomic force microscopy (AFM) after removal of thecarbon protection film 6 revealed that, whereas asperity to the extentof several dozens nanometers was created over the entire surface of thesilicon carbide wafer in a case without the carbon protection film 6,asperity less than one nanometer was created in the case with the carbonprotection film 6 according to the invention, whereby the effect of thefilm was verified. Moreover, since the carbon protection film 6 that isof high purity with few contaminants is formed, the silicon carbidesemiconductor device does not become contaminated. Furthermore, sincethe carbon protection film 6 is formed over the entire surface of thesilicon carbide wafer, unbalanced thermal stress created therein duringthe annealing treatment is relieved, so that distortion created in thesilicon carbide wafer is reduced. For that reason, crystal defectscreated in the silicon carbide wafer do not increase.

Next, a gate oxide film 7 made of silicon dioxide (SiO₂) is formed onone surface of, specifically, on an ion-implanted side of the siliconcarbide wafer from which side the carbon protection film 6 has beenremoved by thermal oxidation as shown in FIG. 4. The gate oxide film 7thus formed in this process is a thermal oxidation film.

Next, a polysilicon film is temporarily formed on the gate oxide film 7by a CVD method. Then, a gate electrode 8 is formed, after anunnecessary portion of the polysilicon film is removed by a wet etchingmethod or a dry etching method such as a reactive ion etching (RIE)using a resist as a mask for the gate electrode, as shown in FIG. 5.

Next, an interlayer insulation film 9 made of silicon dioxide (SiO₂) isformed on the gate oxide film 7 and the gate electrode 8 by a CVD methodusing a TEOS gas, as shown in FIG. 6. The interlayer insulation film 9formed in this process is a TEOS oxide film.

Next, the interlayer insulation film 9 and the gate oxide film 7 areremoved by a wet etching method or a dry etching method such as a RIEusing a resist as a mask for exposing the contact region 5 and part ofthe source region 4, as shown in FIG. 7. After that, the resist is alsoremoved.

Next, a conductive film is formed on the contact region 5 and the partof the source region 4 that have been exposed by removing the interlayerinsulation film 9 and the gate oxide film 7, by a PVD method such as asputtering method. After that, a source electrode 10 (a first mainelectrode) is formed on the contact region 5 and the part of the sourceregion 4 by removing an unnecessary portion of the conductive filmformed on the interlayer insulation film 9 by a wet etching method or adry etching method such as RIE using a resist as a mask for the sourceelectrode, as shown in FIG. 8. The source electrode 10 is electricallyconnected with the contact region 5 and the source region 4. Nickel (Ni)or aluminum (Al) is given as a material for the source electrode 10.

Lastly, a drain electrode 11 (a second main electrode) made of aconductive film is formed on the silicon carbide wafer surface oppositeto the source electrode by a PVD method such as a sputtering method, asshown in FIG. 8. Nickel (Ni) or aluminum (Al) is given as a material forthe drain electrode 11.

Through the processes described above, the main part of the power MOSFETis completed that is the silicon carbide semiconductor device accordingto Embodiment 3 of the invention.

A method of manufacturing a silicon carbide semiconductor device,according to Embodiment 3 of the invention includes a process of forminga carbon protection film of a predetermined thickness over the entiresurface of a silicon carbide wafer having been ion-implanted with animpurity, by a CVD method that deposits a film by pyrolyzing anoxygen-containing hydrocarbon gas. Since the carbon protection film 6that is of high quality and high purity with few contaminants canthereby be obtained, the silicon carbide semiconductor device can beprevented from being contaminated. Moreover, since unbalanced thermalstress created in the silicon carbide wafer can be suppressed, crystaldefects, associated with the thermal stress, in the silicon carbidewafer can also be prevented from increasing. Therefore, the method ofmanufacturing a silicon carbide semiconductor device can be obtainedthat achieves improvements in its quality stability and manufacturingyields.

Moreover, the carbon protection film deposited from an oxygen-containinghydrocarbon gas has fewer defects than that deposited from a generalhydrocarbon gas containing no oxygen.

This is due to the fact that activated oxygen produced during thepyrolysis of the oxygen-containing hydrocarbon gas at a high temperaturereacts with the depositing amorphous carbon to produce carbon monoxide,which contributes to reducing defects in the carbon protection film.

Moreover, using an oxygen-containing hydrocarbon gas for forming acarbon protection film decreases production of unreacted hydrocarbonduring formation of the carbon protection film by a CVD, which reducesadhesion of the unreacted carbon and higher hydrocarbon film to theinside of the deposition apparatus. Therefore, an effect is also broughtabout that improves maintainability of the deposition apparatus.

Although the case of using a lower alcohol such as ethanol and methanolas an oxygen-containing hydrocarbon gas is explained as an example inEmbodiment 3, a gas consisting of oxygen, carbon, and hydrogen can beused as an oxygen-containing hydrocarbon gas: for example, a higheralcohol such as cetanol, a hydroxyl acid, a carboxylic acid, a ketone,an aldehyde, a phenol, an ester, or an ether may also be used byevaporating it. For such case, a manufacturing method equivalent to thatof Embodiment 3 is employable and can bring about the same effect. It isnoted that, in practical use, a higher oxygen-containing hydrocarbon israther undesirable; a lower one containing ten or less carbons in amolecule is desirable, and additionally a lower one having a highervapor pressure is more desirable. From that point of view, it can besaid that a lower alcohol such as ethyl alcohol and methyl alcohol isdesirable.

Embodiment 4

While the explanation is made in Embodiment 3 on formation of the carbonprotection film 6 using an oxygen-containing hydrocarbon gas such asethanol and methanol, in a case of using a cyclic ether compound such astetrahydrofuran (THF) as an oxygen-containing hydrocarbon gas, a carbonprotection film 6 is formed under slightly different conditions. Aprocess different from that in the method of manufacturing the siliconcarbide semiconductor device in Embodiment 3, that is, formation of thecarbon protection film 6 using tetrahydrofuran will be explained belowwith reference to FIGS. 1 through 8 and FIG. 10. The other processes inthe method of manufacturing the silicon carbide semiconductor device arethe same as those explained in Embodiment 3; their explanations aretherefore omitted.

After a silicon carbide wafer has been processed up to the stage shownin FIG. 2 in Embodiment 3, the carbon protection film 6 is formed overthe entire surface of the silicon carbide wafer using tetrahydrofuran asan oxygen-containing hydrocarbon gas by a CVD method or the like, asshown in FIG. 3.

To be more specific, while an inert gas such as argon (Ar) is beingintroduced as a carrier gas to flow into the deposition furnace 32performing a CVD, the inside of the deposition apparatus is heated at atemperature ranging from 850° C. to 1000° C. using the heater 34 underreduced pressure. The heating time is approximately twenty minutes.After that, a pressurized tetrahydrofuran is fed gradually into anevaporator from a feeder, which are not shown, to be evaporated in theevaporator at 120° C., and then introduced into the deposition furnace32 through the gas inlet pipe 31. In order to prevent condensation, thepipes from the evaporator to the deposition furnace 32, including thegas inlet pipe 31, are heated above 100° C. by heaters, which are notshown, provided around the pipes. Tetrahydrofuran needs to be pyrolyzedand deposited under a pressure below 1.33×10⁴ Pa (100 Torr) in thedeposition furnace 32. More desirably, the pressure therein is set to7×10³ Pa (52.5 Torr), which enhances the deposition rate. In this way,the oxygen-containing hydrocarbon gas introduced into the depositionfurnace 32 is pyrolyzed therein, to form the carbon protection films 6that is of high quality and high purity with few contaminants to apredetermined thickness over the entire surface of each of a pluralityof silicon carbide wafers placed on the substrate holder 35. Thepredetermined thickness here, as mentioned before, is preferable to beone nanometer, above which the effect of the protection film is broughtabout, to one thousand nanometers, under which cracks due to the thermalload imposing the silicon carbide wafer and to the temperaturedifference arising therein are not produced in the protection film. Itis more preferable to be ten to five hundred nanometers, in which rangethe thickness is easy to control. In addition, the high quality impliesan advanced state of graphitization. After the carbon protection film 6of the predetermined thickness is formed, the feed of thetetrahydrofuran from the feeder is stopped, and the inside thedeposition furnace 32 is cooled down to a temperature at which thesilicon carbide wafers can be removed therefrom.

Thereby, the carbon protection film 6 can be formed that brings aboutthe same effect as that of Embodiment 3. Processes subsequent to thatare the same as the annealing treatment and those subsequent thereto inEmbodiment 3.

Embodiment 5

According to Embodiment 1 through Embodiment 4, after the carbonprotection film 6 has been formed over the entire surface of each ofsilicon carbide wafers in the deposition apparatus, the silicon carbidewafers are transferred into the annealing apparatus. For that reason,the carbon protection film 6 formed on the silicon carbide wafersurfaces may be subject to peel or partial damage due to rubbing duringthe transfer, which gives cause for concern that the silicon carbidesemiconductor device decreases in its quality and manufacturing yields.

A method of preventing such damages is to share a substrate holder 35between the deposition apparatus that deposits the carbon protectionfilm 6 over the entire surface of the silicon carbide wafers and theannealing apparatus that anneals the silicon carbide wafers having beendeposited with the carbon protection film 6 thereon.

In other words, the substrate holder 35 that enables the carbonprotection film 6 to be deposited over the entire surface of each of theplurality of silicon carbide wafers and enables the wafers to be placedon the holder for a batch operation, is provided usable in bothdeposition and annealing apparatuses shown in Embodiment 1 throughEmbodiment 4.

FIG. 11 shows an example of a typical annealing apparatus. In FIG. 11,an annealing apparatus 40 is provided with an annealing furnace 41; agas inlet pipe 42 for introducing an inert gas such as argon into theannealing furnace 41; and an exhaust gas pipe 43 for exhausting gaseswithin the annealing furnace 41 therethrough; the substrate holder 35 onwhich the plurality of silicon carbide wafers can be placed and isusable also for the deposition apparatus; a heater 44 such as a carbonheater, disposed so as to surround the substrate holder 35; heatspreaders 45, which are made of silica, ceramics, or the like, disposedat least around the heater 44, for uniformly transferring heat from theheater 44 to the silicon carbide wafers; and a coil 46 disposed aroundthe periphery of the annealing furnace 41, for heating the heater 44 byinduction.

Using the annealing apparatus 40 thus configured, after exhaust of gaseswithin the annealing furnace 41, argon gas is introduced though the gasinlet pipe 42. Then, heat from the heater 44 induction-heated by thecoil 46 is uniformly spread by the heat spreaders 45, so that thesilicon carbide wafers placed on the substrate holder 35 areisothermally heated in the argon atmosphere set at a predeterminedpressure, whereby a desired annealing treatment is performed. Inaddition, the substrate holder 35 used in the annealing treatment hasbeen used in the deposition apparatus.

In this way, by using the substrate holder 35 in common between thedeposition and the annealing processes, silicon carbide wafers placed onthe substrate holder 35 can be transferred from the deposition apparatusto the annealing apparatus without changing the substrate holder.Therefore, the carbon protection film 6 formed on each silicon carbidewafer surface is not subject to peel or partial damage due to rubbingduring the transfer, achieving further improvements in quality stabilityand manufacturing yields of the silicon carbide semiconductor device.Moreover, since the insides of the deposition and the annealingapparatuses become less contaminated, production efficiency can also beenhanced.

It is noted that, in each embodiment, the silicon carbide wafer issupported by the substrate holder during deposition of the carbonprotection film 6. For that reason, the carbon protection film 6 is notformed or is difficult to form on portions where the substrate holdertouches, of the silicon carbide wafer. However, such contact portionsare smaller area with respect to the whole area of the silicon carbidewafer, which does not affect the effect of the invention at all.Therefore, “the silicon carbide wafer that is formed with the carbonprotection film 6 over the entire surface thereof”, referred to in theinvention, also includes a silicon carbide wafer having portions, wherethe substrate holder touched, on which the carbon protection film 6 isnot formed or is formed in a thinner film due to difficulty indepositing.

1. A carbon film deposition apparatus comprising: an evaporator forevaporating an oxygen-containing hydrocarbon; a gas inlet pipe forintroducing the oxygen-containing hydrocarbon gas evaporated in theevaporator; and a deposition furnace for depositing a carbon protectionfilm over all surfaces of a wafer by pyrolyzing the oxygen-containinghydrocarbon gas introduced through the gas inlet pipe.
 2. The carbonfilm deposition apparatus set forth in claim 1, wherein a pipe includingthe gas inlet pipe provided from the evaporator to the depositionfurnace is heated by a heater.
 3. The carbon film deposition apparatusset forth in claim 1, wherein the deposition furnace is provided with asubstrate holder that holds the wafer by supporting its periphery atthree points.
 4. The carbon film deposition apparatus set forth in claim1, wherein the oxygen-containing hydrocarbon gas is any of methylalcohol, ethyl alcohol, cetanol, hydroxyl acid, carboxylic acid, keton,aldehyde, phenol, ester, ether, and tetrahydrofuran.
 5. A carbon filmdeposition apparatus comprising: a gas inlet pipe for introducing ahydrocarbon gas; a gas pyrolysis furnace for pyrolyzing the hydrocarbongas introduced through the gas inlet pipe; and a deposition furnace fordepositing a carbon protection film over all surfaces of a wafer byintroducing the hydrocarbon gas pyrolyzed in the pyrolysis furnace. 6.The carbon film deposition apparatus set forth in claim 5, wherein thegas pyrolysis furnace is provided with a heater and a heat spreader forheating uniformly the inside of the gas pyrolysis furnace by receivingheat from the heater.
 7. The carbon film deposition apparatus set forthin claim 6, wherein the heat spreader is made of silica glass orceramics.
 8. The carbon film deposition apparatus set forth in claim 5,wherein a temperature in the gas pyrolysis furnace is set higher thanthat in the deposition furnace.